The present invention relates generally to the art of metallurgy and, more particularly, to high speed tool steels.
High speed steels are composite materials that contain a variety of alloy carbide particles in an iron base plus, depending on the heat treatment, various atomic arrangements of iron carbon in the form of austenitic, ferritic, bainltic and martensitic structures. Various carbide forming elements such as, for example, chromium, molybdenum, tungsten and vanadium, are constituents of high speeds. Infrequently, niobium and titanium are used as additional carbide forming elements. These above enumerated elements are found combined as carbides as the result of ledeburitic and eutectoid reactions as the molten alloy solidifies and transformation as the temperature drops. Silicon is normally present and higher levels may be added to the alloy to increase attainable hardness.
Because of the high temperatures produced during machining more difficult materials, the retention of the critical cutting surfaces is related to the hardness of the tool. The ability of the tool to retain its hardness is assessed by the hardness of the tool at elevated temperatures. Retention of the hardness can be measured by testing the steel at a given temperature or heating the steel for a prolonged time at a given temperature then measuring the steel's retention of hardness at room temperature when the tool cools down. The present invention improves the hot hardness properties of high speed steel without the use of cobalt or very high tungsten and/or molybdenum combinations. Cobalt is not only expensive but its supply is irregular and the use of very high tungsten and molybdenum combinations produce steels that are difficult to hot work without utilizing costly powder metallurgy methods.
The present invention provides a family of high speed steel compositions that have the capability of achieving high hardness upon proper hardening and retaining a significant portion of that property at temperatures commonly encountered by cutting tools such as drills, taps and reamers. These steels are also useful in operations that require high hardness at more moderate to room temperature operations such as punches and thread forming tools.